The present invention relates to a selector element for memory applications, and more particularly, to embodiments of a two-terminal selector element having multiple threshold and holding voltages.
A resistance-based memory device normally comprises an array of memory cells, each of which includes a memory element and a selector element coupled in series between two electrodes. The selector element functions like a switch to direct current or voltage through the selected memory element coupled thereto. The selector element may be a three terminal device, such as transistor, or a two-terminal device, such as diode or Ovonic threshold switch. Upon application of an appropriate voltage or current to the selected memory element, the electrical property of the memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
FIG. 1 is a schematic circuit diagram of a memory array 30, which comprises a plurality of memory cells 32 with each of the memory cells 32 including a bi-directional two-terminal selector element 34 coupled to a resistance-based memory element 36 in series; a first plurality of parallel wiring lines 38 with each being coupled to a respective row of the memory elements 36 in a first direction; and a second plurality of parallel wiring lines 40 with each being coupled to a respective row of the selection elements 34 in a second direction substantially perpendicular to the first direction. Accordingly, the memory cells 32 are located at the cross points between the first and second plurality of wiring lines 38 and 40.
The resistance-based memory element 36 may be classified into at least one of several known groups based on its resistance switching mechanism. The memory element of Phase Change Random Access Memory (PCRAM) may comprise a phase change chalcogenide compound, which can switch between a resistive phase (amorphous or crystalline) and a conductive crystalline phase. The memory element of Conductive Bridging Random Access Memory (CBRAM) relies on the statistical bridging of metal rich precipitates therein for its switching mechanism. The memory element of CBRAM normally comprises a nominally insulating metal oxide material, which can switch to a lower electrical resistance state as the metal rich precipitates grow and link to form conductive paths upon application of an appropriate voltage. The memory element of Magnetic Random Access Memory (MRAM) typically comprises at least two layers of ferromagnetic materials with an insulating tunnel junction layer interposed therebetween. When a switching current is applied to the memory element of an MRAM device, one of the ferromagnetic layers will switch its magnetization direction with respect to that of the other magnetic layer, thereby changing the electrical resistance of the element.
A magnetic memory element normally includes a magnetic reference layer and a magnetic free layer with an electron tunnel junction layer interposed therebetween. The magnetic reference layer, the electron tunnel junction layer, and the magnetic free layer collectively form a magnetic tunnel junction (MTJ). Upon the application of an appropriate current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction of the magnetic reference layer. The electron tunnel junction layer is normally made of an insulating material with a thickness ranging from a few to a few tens of angstroms. When the magnetization directions of the magnetic free and reference layers are substantially parallel or oriented in a same direction, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ. Conversely, the electrical resistance of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel or oriented in opposite directions. The stored logic in the magnetic memory element can be switched by changing the magnetization direction of the magnetic free layer between parallel and anti-parallel with respect to the magnetization direction of the reference layer. Therefore, the MTJ has two stable resistance states that allow the MTJ to serve as a non-volatile memory element.
Based on the relative orientation between the magnetic reference and free layers and the magnetization directions thereof, an MTJ can be classified into one of two types: in-plane MTJ, the magnetization directions of which lie substantially within planes parallel to the same layers, or perpendicular MTJ, the magnetization directions of which are substantially perpendicular to the layer planes.
FIG. 2 shows an intrisic current-voltage (I-V) response plot for a conventional two-terminal selector device without an external load (e.g., memory element) coupled thereto. The I-V response curve 50 shows the magnitude of electric current passing through the two-terminal selector device as the voltage applied thereto varies. Initially, the current gradually increases with the applied voltage from zero to near a threshold voltage, Vth. At or near Vth, the current rapidly increases and exhibits a highly non-linear behavior. As the voltage continues to increase beyond Vth, the current increase becomes gradual again until reaching Vp, which is the programming voltage for the whole memory cell when a memory element is coupled to the selector device. The current response behaves like a step function as the applied voltage increases from zero to Vp with the sharp increase occurring at or near Vth, which is about 60-80% of Vp in order to minimize current leakage.
Similarly, to read or sense the resistance state of a memory element, the voltage of the selector device needs to be raised to above Vth first to turn on the selector device. However, for some types of memory elements, such as MTJ, the high current level associated with Vth may cause the unintended switching of the resistance state during a read operation, commonly known as “read disturbance.” To prevent the read disturbance of MTJ, the sense current needs to be kept low, preferably about 10% of the programming current.
For the foregoing reasons, there is a need for a bi-directional two-terminal selector element that minimizes the read disturbance of memory elements and that can be inexpensively manufactured.